1. Technical Field
The technical field relates to directional sound systems and more particularly to a sound system for projecting a high intensity, wide spectrum sound beam, particularly in a compressible medium such as air.
2. Description of the Problem
High intensity sound beams in air have long been produced using parabolic reflectors. Meyer et al., in U.S. Pat. No. 5,821,470 described a Broadband Acoustical Transmitting System based on a parabolic reflector incorporating two loudspeaker transducers. One transducer was spaced from the reflector, forward along the intended axis of propagation of sound at the focal point of the reflector. This transducer was horn loaded and oriented to propagate sound backward along the radiant axis and into the reflector for reflection in a collimated beam. The horn loaded transducer was intended to handle the higher frequency components of the overall field. A second transducer for low frequency components was located opposed to the horn loaded transducer on the radiant axis, preferably flush mounted in the parabolic reflector and oriented for forward propagation of sound. At this location the low frequency transducer would derive relatively little benefit from the focusing aspects derived from the parabolic shape of the reflector, though the reflector would serve as a baffle.
Volume can be increased in general by using an increasing number of transducers operating on the same input signal. Generally, a simple parabolic reflector such as employed by Meyer does not readily allow the use a large number of transducers operating on the same input because of the inability to place more than one transducer at the focal point of the reflector. U.S. Pat. No. 4,796,009 to Biersach teaches a high volume sound projector where a potentially large plurality of loudspeakers or drivers are coupled to emit into a acoustical impedance chamber having a restricted output port. The output port opens into the throat of a horn.
Another technique for combining the outputs of a number of drivers is to place them in an array where the distances between loudspeakers result in the sound fields produced by the drivers produce zones of constructive and destructive interference. The resulting sound field can be shaped and steered by adjusting the phase relationship of the outputs of the transducers. Another sound reinforcement system which can accept inputs from a large plurality of transducers arranged in a line array and which employs a reflecting surface having some attributes of a parabolic dish was described in U.S. Patent Application Publication 2008/0121459 for an Acoustic Energy Projection System by the present inventor which is incorporated herein by reference. One embodiment of Publication 2008/0121459 related to a sound generation and projection apparatus based on a radiator having a conically shaped and parabolically sculpted reflecting surface. The shaped reflecting surface defined a set of equivalent acoustic input locations. The set of locations typically lay in a circle or ring centered on the radiant axis and encircling the reflecting surface just forward from its base.
The set of equivalent acoustic input locations had non-zero circumference centered on the forward radiant axis. Sound sources located acoustically “close” to one another at the acoustic locations would form a distributed sound source which operates as a continuous source below a cut off frequency. The sound source was, in effect, a curved, or even closed loop line array of loudspeakers.
Horns function as impedance matching devices, that is a type of transformer, providing a high impedance to a driver at its input end (the throat) and expanding in cross-sectional area to meet the low impedance of free air at the radiating end (mouth) of the horn. Substantial efficiency gains are possible using horns, however, horn effectiveness depends upon relating dimensions of the horn to the sound frequency spectrum to be radiated. One of these relationships is the one between the area of the mouth (usually expressed in terms of circumference of the mouth for a given mouth shape) and wavelength. For a horn having a circular cross-section the ratio of the mouth circumference to wavelength should exceed 1 for the longest wavelength in the sound spectrum to be radiated. By inference, a given horn ceases to provide any substantial impedance matching capacity to shorter wavelengths in the spectrum at some point along the horn short of the mouth. Thus horn loading the transducers of U.S. Patent Application Publication 2008/0121459 would produce efficiency gains.
However, accommodating horns in a projector of limited size becomes difficult if low acoustic frequencies are important, particularly for some flare types, due to the increasing length of the horn used to achieve most of the available gain.